1. Field of the Invention
The invention relates to a method for filling compressed-gas containers, in particular in airbag systems, with a gas mixture.
2. Description of the Related Art
Vehicle airbags are increasingly using new types of gas generators, which in the event of an accident inflate the airbag within a few milliseconds. At present, three types of gas generators are in use:                chemical generators, in which the gas is generated by the reaction of a chemical solid with ambient air;        what are known as hybrid generators, which comprise a combination of solid fuel and compressed-gas packing, and        pure gas generators with a high-pressure gas storage system at pressures of up to 700 bar at 15° C.        
The gas generators, which are filled with various gases, cause huge technical problems in production, both in terms of their production and in terms of filling at pressures of up to 1000 bar. These pressures are required in particular for rapid filling, on account of the heat of compression, in order to introduce the precisely predetermined gas masses. These are of crucial importance to the subsequent inflation characteristics of the airbag.
Examples of gases used include argon, oxygen, nitrogen, dinitrogen monoxide (laughing gas, nitrous oxide), both in the form of ultra-pure gases and in the form of gas mixtures of these components.
Gas-filled gas generators require:                1. Filling pressures of up to 1000 bar (P(T)) for higher storage density or more compact dimensions.        2. Accurate and exact filling quantity determination at high pressures.        3. Rapid filling, since it is the filling which determines the cycle times.        4. The operation must be highly reproducible.        
Very expensive and complicated piston or diaphragm compressors are required in order to generate the very high pressures. This entails high investment costs, high operating and maintenance costs. In addition, a downstream gas supply which is correspondingly complex and expensive is required for these pressures.
With increasing pressures, on account of the heat of compression and the uneven temperature distribution in the pressure vessel, the inaccuracy of the precise filling quantity determination increases, yet this accuracy is imperative for the generator to subsequently function as defined.
With increasing pressures, it becomes technically more difficult and complex to achieve fast filling times. There is a direct relationship between filling time and warming during the filling operation, i.e. the more quickly the filling is carried out, the more the gas temperature rises, with the result that the filling pressure has to be increased still further in order to achieve the exact quantity of gas for 15° C. or another defined temperature.
For the abovementioned reasons, the reproducibility is becoming more difficult and entails complex QA measures, such as for example weighing of the filled vessels for accurate filling quantity determinations. At the same time, a considerable rise in the scrap rate is expected at higher pressures. This in turn leads to an adverse effect on the economics of the overall process and therefore to higher production costs.